Endangered Species Recovery Program
Taxonomy.-- The San Joaquin antelope squirrel is one of five species of antelope squirrels. Members of the genus Ammospermophilus are confined to desert, arid steppe, and open shrubland communities in the southwestern United States and northern Mexico. Ammospermophilus nelsoni was described by Merriam (1893) as a member of the genus Spermophilus; the type specimen was from Tipton, Tulare County, California. A. nelsoni also has been placed in the genus Citellus. Taylor (1916) distinguished the northern populations as a subspecies, A. nelsoni amplus, but A. nelsoni currently is considered to be monotypic (Hall 1981, Hafner 1981).
Description.-- The San Joaquin antelope squirrel (Figure 56) has a typical ground-squirrel shape: tiny, rounded ears, and streamlined, fusiform (spindle- shaped) body with relatively short legs and tail. The tail has laterally projecting thick fringes of hairs, and is usually held cocked or curled over the back. The upper parts are colored buffy-tan with a light stripe along the sides. The underside of the tail is light grayish or whitish. Individuals range from about 218 to 240 millimeters (8.5 to 9.4 inches) in length (Hall 1981), and adults weigh from about 130 to 170 grams (4.6 to 6.0 ounces) (Williams 1980).
Identification.-- The San Joaquin antelope squirrel can be distinguished from the co-occurring California ground squirrel by much smaller size; shorter, less bushy tail with a flattened shape rather than the bottle-brush shape of the California ground squirrel; and the presence of a light-colored stripe along the sides of the body. Many people think antelope squirrels are chipmunks, but antelope squirrels lack the light and dark stripes on the face and the light and dark stripes on the back, which are characteristic of western chipmunks (Tamias spp.).
Figure 56. Illustration of a San Joaquin antelope squirrel (Ammospermophilus nelsoni) . Drawing by Deborah Basey (© by D.F. Williams).
Historical Distribution.-- The historical distribution of the San Joaquin antelope squirrel included the western and southern portions of the Tulare Basin, San Joaquin Valley, and the contiguous areas to the west in the upper Cuyama Valley and on the Carrizo and Elkhorn Plains (Figure 57). They ranged from western Merced County on the northwest, southward along the western side of the San Joaquin Valley to its southern end. They were distributed over the floor of the San Joaquin Valley in Kern County and along the eastern edge of the Valley northward to near Tipton, Tulare County (Hall 1981, Williams 1980). San Joaquin antelope squirrels range in elevation from about 50 meters (165 feet) on the San Joaquin Valley floor to about 1,100 meters (3,600 feet) in the Temblor Mountains. Antelope squirrels are not common above about 800 meters (2,600 feet) on the ridges and plains west of the San Joaquin Valley proper (Williams 1980, D.F. Williams unpubl. data). The area encompassed by the distribution records prior to cultivation was approximately 1,398,600 hectares (3,456,000 acres). Grinnell and Dixon (1918) wrote that San Joaquin antelope squirrels were unevenly distributed and occurred in abundance in only a few localities; one ws in the Lokern and Elk Hills region of western Kern County.
Current Distribution.-- Extant, uncultivated habitat for San Joaquin antelope squirrels was estimated in 1979 to be 275,200 hectares (680,000 acres) (Williams 1980). This estimate encompassed the land occupied by towns, roads, canals, pipelines, strip mines, airports, oil wells, and other developments. None of the best habitat described by Grinnell and Dixon (1918) remained. Only about 41,300 hectares (102,000 acres) was rated as fair to good quality, supporting from 3 to 10 antelope squirrels per hectare (1 to 4 per acre). Antelope squirrels had been nearly eliminated from the floor of the Tulare basin, and existed mainly in marginal habitat in the mountainous areas bordering its western edge. Substantial populations were found only in and around Lokern and Elk Hills in western Kern County, and on the Carrizo and Elkhorn Plains in eastern San Luis Obispo County.
Since 1979, San Joaquin antelope squirrels have disappeared from many of the smaller islands of habitat on the Valley floor, including Pixley National Wildlife Refuge, Tulare County; Alkali Sink and Kerman Ecological Reserves, Fresno County; and several areas within the Allensworth Conceptual Area of Tulare and Kern Counties (Williams 1980, Harris and Stearns 1991, D.F. Williams unpubl. observ., Endangered Species Recovery Program unpubl. data).
Food and Foraging.-- San Joaquin antelope squirrels are omnivorous. The amount and type of food consumed are mostly dependent upon availability. The squirrels eat green vegetation, fungi, and insects more often than seeds, even when seeds are relatively abundant (Hawbecker 1975, Harris 1993). Vegetation and seeds of filaree and red brome are the main food plants (Hawbecker 1953). Insects, principally grasshoppers, are eaten regularly when available. Seeds of shrubs such as ephedra and saltbush also are staples. Seeds and insects may be necessary in the diet as sources of protein. When seeds and grasshoppers are scarce, antelope squirrels eat harvester ants (Hawbecker 1975). During spring, especially during severe drought, San Joaquin antelope squirrels eat large quantities of ovaries and developing seeds of ephedra (D.F. Williams unpubl. observ.).
Reproduction and Demography.-- The breeding period for San Joaquin antelope squirrels is late winter through early spring. There is only one breeding period per year, coinciding with the time of year when green vegetation is present (Hawbecker 1953, 1958). Young squirrels do not breed their first year (Hawbecker 1975). Testes of males begin to enlarge in September or October and reach maximum size by November or December, long before the ovaries of females begin to develop (Best et al. 1990). Copulation and conception usually take place in February or March. By the end of March, testes begin to regress in size and maintain a minimum size of about 4 to 8 millimeters (0.2 to 0.3 inch) through the summer. All males are not reproductively active at the same time; some males may have enlarged testes in May (Hawbecker 1975).
Gestation lasts about 26 days. Embryos are present in late January, but development is concentrated in February and early March. Embryos range in number from 6 to 11, with an average of 8.9 (Hawbecker 1975).
Young are born between March and April and are first seen above ground when about 30 days of age (Williams and Tordoff 1988). Young are weaned beginning in late April; the last young are weaned in mid- or late-May (Hawbecker 1975).
Timing, nature, and distance of dispersal are poorly documented; Hawbecker (1975) noted that weaned young were still together in late May. Williams and Tordoff (1988) noted at least some family groups were still together in mid-July. Young San Joaquin antelope squirrels on the Elkhorn Plain Ecological Reserve had a mortality rate of about 70 percent during their first year of life, and adults had a mortality rate from about 50 to 60 percent (Williams and Tordoff 1988).
Behavior and Species Interactions.-- San Joaquin antelope squirrels live in burrows, either of their own construction or ones dug by kangaroo rat. They may also take over and enlarge burrows dug by Heermanns kangaroo rats (Grinnell and Dixon 1918, Hawbecker 1947, 1953, Williams 1980). Hawbecker (1947, 1953) believed that antelope squirrels were dependent upon kangaroo rats to dig burrows because the many burrows examined by him all seemed to have been dug by kangaroo rats. In contrast, Grinnell and Dixon (1918) believed that they dug their own burrows. Burrows vary in complexity and length, but generally have two to six openings and are between about 30 and 50 centimeters (12 to 20 inches) deep. Favored locations for burrows are in the side of an arroyo, the berm of an unimproved road, or under shrubs (Williams 1980).
Antelope squirrels make use of both shrubs and burrows of giant kangaroo rats as sites of refuge from predators as they move across their home ranges. They also regularly retreat to the shade of shrubs to avoid the heat of the sun and to dump excess body heat to the cooler, shaded ground. Burrows of giant kangaroo rats may serve the same purpose (Williams et al. 1988, Williams and Kilburn 1992).
California ground squirrels displace San Joaquin antelope squirrels and may even restrict the range of the antelope squirrel (Taylor 1916, Harris and Stearns 1991). Hawbecker (1953) noted that the range of the San Joaquin antelope squirrel may be determined, to some degree, by the range of co-occurring kangaroo rat species. The range of giant kangaroo rats most nearly coincides with that of the San Joaquin antelope squirrel, but their microhabitats generally differ in many areas. Populations of Heermanns kangaroo rats are common in most areas where antelope squirrels are found. San Joaquin kangaroo rats also occur in the same areas as San Joaquin antelope squirrels, but these kangaroo rats are much smaller; their small-diameter burrows would have to be enlarged considerably before antelope squirrels could use them (Williams 1980).
San Joaquin antelope squirrels probably compete with kangaroo rats for seeds, especially those of grasses and forbs, and, to a lesser extent, green herbaceous material. The extent to which kangaroo rats eat insects, an important staple for antelope squirrels, is unknown, but insects are probably only a minor part of their diets. Species of birds are probably the main competitors of antelope squirrels for insects (Williams and Tordoff 1988). San Joaquin antelope squirrels are prey for a variety of animals: hawks, falcons, eagles, snakes, kit foxes, coyotes, badgers and probably other predators (Williams and Tordoff 1988).
Activity Cycle.-- San Joaquin antelope squirrels are primarily diurnal, usually active early or late in the day (Elliot 1904). Activity is reduced when ambient temperatures drop below about 10 degrees Celsius (50 degrees Fahrenheit) (Hawbecker 1958), but on sunny days they have been observed when air temperatures were around 0 degrees Celsius (32 degrees Fahrenheit) (D.F. Williams unpubl. observ.). Activity also is reduced at high ambient temperatures, but the amount and critical temperatures at which activity is curtailed are unclear. On the Elkhorn Plain Ecological Reserve, antelope squirrels were observed at all hours of the day and at ambient temperatures in excess of 42 degrees Celsius (108 degrees Fahrenheit) during July and August (Williams and Tordoff 1988). In contrast, Hawbecker (1958) noted that squirrels occasionally ventured into the hot sun only for short periods. They are active above ground for extensive periods during the day in the spring when temperatures are generally between about 20 to 30 degrees Celsius (68 to 86 degrees Fahrenheit).
Habitat and Community Associations.-- San Joaquin antelope squirrels live in relatively arid annual grassland and shrubland communities in areas receiving less than about 23 centimeters (10 inches) of mean annual precipitation. They are most numerous in areas with a sparse-to-moderate cover of shrubs such as saltbushes, California ephedra, bladderpod, goldenbushes, matchweed, and others. Shrubless areas are only sparsely inhabited, especially where giant kangaroo rats are not present or not common.
Hawbecker (1953) believed that most antelope squirrls found in shrubless areas were nonbreeders. Yet, on the Carrizo Plain Natural Area antelope squirrels are widespread; permanent populations are found over thousands of acres without shrubs (Harris and Stearns 1991, D.F. Williams, unpubl. observ.). Grinnell and Dixon (1918) and Hawbecker (1953) observed that San Joaquin antelope squirrels rarely occurred on the Valley floor in areas with alkaline soils supporting halophytes such as iodine bush and spiny saltbush. Highly alkaline soils on the Valley floor typically have water tables within a few centimeters to a meter (1 to 40 inches) or so from the surface, perhaps limiting habitation. Steep slopes and broken, rocky, upland terrain are also scarcely inhabited (Williams 1980).
San Joaquin antelope squirrels require areas free from flooding where they can place ground burrows. Soils must be friable. Substantial colonies investigated by Hawbecker (1953) were almost always confined to loam and sandy-loam soils with moderate amounts of soluble salts, but soils with a wide range of textures are used (Williams 1980). In shrubless areas, and many areas with sparse shrub cover, San Joaquin antelope squirrels are associated with giant kangaroo rats, and they also live in burrow systems made by giant kangaroo rats (Williams and Tordoff 1988, Williams et al. 1993b, D.F. Williams unpubl. observ.).
In the southern and western San Joaquin Valley, San Joaquin antelope squirrels are associated with open, gently sloping land with shrubs. Typical vegetation includes saltbushes and ephedra (Hawbecker 1975). Near Panoche, San Benito County, at an elevation of about 360 meters (1,200 feet), they are associated with such plants as California ephedra, California juniper, matchweed, one-sided bluegrass (Poa secunda ssp. secunda), red brome, and red-stemmed filaree (Hawbecker 1958). Near Los Banos, Merced County, and near Mendota, Fresno County, the habitat is mostly devoid of brushy cover (Hawbecker 1947).
Reasons for Decline.-- Loss of habitat to agricultural developments, urbanization, and petroleum extraction is the principal factor threatening San Joaquin antelope squirrels. Use of rodenticides for control of ground squirrels and San Joaquin antelope squirrels was reported by Grinnell and Dixon in 1918. Use of insecticides to control leafhoppers and other insects might impact antelope squirrels negatively by temporarily reducing the abundance of insects, an important source of food and moisture during summer.
Threats to Survival.-- The processes of habitat loss and fragmentation are expected to continue on a much smaller scale than in the past, but the direct and indirect effects of these processes are expected to accelerate the decline of the species. Though one of the two largest and most important habitat areas, the Carrizo Plain Natural Area, is now mostly in public ownership, potential protection is tenuous for the species in the equally important population of the Lokern-Elk Hills area of western Kern County. The sale of Naval Petroleum Reserve #1 in Elk Hills to private interests (Henry 1995a, 1995b) could represent a threat to the San Joaquin antelope squirrel if rates of exploration and production are increased.
Another threat to San Joaquin antelope squirrels on private land may be the long-term effects of excessive grazing by livestock. Elimination of shrubs and soil erosion resulting from heavy use of rangeland communities by livestock can degrade their carrying capacities for most member species. First affected are those species dependent upon the plants most palatable and vulnerable to grazing and browsing by livestock. San Joaquin antelope squirrels appear to maintain good population densities on moderate-to-severely degraded rangelands where shrubs such as ephedra are common, but it is doubtful that they could maintain viability indefinitely unless the processes of overgrazing and resulting soil erosion were halted. Substantial soil erosion has occurred on both public and private lands throughout the historical geographic range of the species (Williams et al. 1993b, D.F. Williams unpubl. observ.). Rangeland conditions in the egion have deteriorated over the last several decades, and deep gully erosion is accelerating, even in areas where livestock grazing has been curtailed or reduced.
Conservation Efforts.-- The San Joaquin antelope squirrel was designated a threatened species by the State of California in 1980 (CDFG 1980). The San Joaquin antelope squirrel was removed as a Category 1 candidate for Federal listing in 1995 (USFWS 1995b), and is now considered a species of concern (USFWS 1996).
San Joaquin antelope squirrels were the target species for the first unit of the Allensworth Ecological Reserve (J. Gustafson pers. comm.), and one of several species benefiting from other mitigation and nonmitigation land protection actions (Table 2). The CDFGs Bird and Mammal Conservation program funded studies on ecology and habitat management of San Joaquin antelope squirrels (Williams et al. 1988) and studies of population survey methods, demography, and distribution (Harris and Stearns 1991). The Biological Resources Division of U.S. Geological Survey is studying effects of roads on San Joaquin antelope squirrels in the Carrizo Plain Natural Area, and interactions between San Joaquin antelope squirrels and giant kangaroo rats (G. Rathbun pers. comm.). The Biological Resources Division also funded a study of food habitats of San Joaquin antelope squirrels (Harris 1993).
Conservation Strategy.-- San Joaquin antelope squirrels in the two largest populations on the Carrizo Natural Area and in western Kern County should be protected by appropriate land uses and habitat management. Ensuring that habitat for San Joaquin antelope squirrels is dedicated to conservation objectives will require purchase of title or easement to some parcels, and protection of habitat on existing public lands in western Kern County. Additional populations need protection, especially in western Fresno and eastern San Benito County, along the fringe of the Valley between Fresno and Kern Counties, and on the Valley floor.
The status of antelope squirrels in the Kettleman Hills and on the remaining islands of habitat in the southern San Joaquin Valley is precarious. Protection and enhancement of habitat in the Semitropic Ridge area of Kern County is important to maintaining a population on the Valley floor. Protecting and restoring habitat in the area including Pixley National Wildlife Refuge and Allensworth Natural Area (this area encompasses all the natural and abandoned farm lands in the Allensworth-Delano area of Tulare and Kern Counties), and reintroducing antelope squirrels to Pixley National Wildlife Refuge is necessary to secure a population in the eastern portions of the Valley. Both habitat restoration and management for San Joaquin antelope squirrels will require additional information derived from scientific investigations.